GEOTROPISM IN PARAMOECIUM 185 



move in a very narrow spiral or nearly straight path. In this 

 condition it is easy to observe that the tendency of the posterior 

 end to sink and bring the animal into a position of vertical 

 equilibrium with anterior end up is of very great effectiveness 

 On the other hand when the animal is stimulated externally in 

 any way the swerving habit entirely annuls the passive sinking 

 tendency of the posterior end and becomes the chief means of 

 orientation. The anterior end can orientate the animal actively, 

 the posterior end only passively Now when a heavily loaded 

 Paramoecium moves upward outside of the region of increased 

 buoyancy, it meets with just such a difficulty in its movements 

 as will cause it to swerve in a broad spiral and until the changed 

 path is downward it will continue to move in that sort of a 

 spiral, which, as Jennings shows, is highly adaptive for bring- 

 ing the animal into a new direction where the stimulus to change 

 will be lacking. 



The movement toward the magnet is the most diffuse feature 

 of the circulation, so that superficially it might be set down as 

 the result of the random movements of the animals. Ocular 

 demonstration of the existence of a definite orientation toward 

 the magnet is apparently rendered difficult for two reasons. 

 First, orientation toward the magnet is accompanied by a gradual 

 orientation into the vertical position of equilibrium by sinking 

 of the posterior end, so that the actual path traversed is a curve. 

 The animals do not appear moving in parallel lines, which is 

 quite a striking feature of the upward streaming. Second, there 

 are two paths of less resistance, toward the earth and toward 

 the magnet and considerable intermingling of the two effects. 

 Evidence of a direct pull of the magnet is best seen upon those 

 animals which may be lying quiescent upon the bottom just 

 within its influence. These often appear to move at first as if 

 dragged like passive objects. 



The remaining feature of the circulation to be considered is 

 the dispersion of the animals after they cease to go upward in 

 the field of the magnet. At some point, which differs with 

 different individuals, they begin to swerve and disperse hori- 

 zontally. The further they move away, of course, the more 

 likely they are to be influenced by gravity more than the magnet 

 and return to the bottom. 



